Bonded metal components having uniform thermal conductivity characteristics and method of making same

ABSTRACT

Cookware having improved uniform heat transfer over the entire cross section thereof, the cookware formed from a multi-layered composite metal having a layer of stainless steel roll bonded at or near the core of the composite. The stainless layer is roll bonded to layers of aluminum which, in turn, is roll bonded to layers of stainless steel or aluminum. The layer of stainless steel adjacent to the cooking range may be a ferromagnetic grade of stainless steel if induction-type heating is desired. The cookware may include a non-stick surface applied thereto.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of co-pending patent application Ser. No. 10/608,898filed Jun. 27, 2003, which is to issue Aug. 9, 2005, as U.S. Pat. No.6,926,971, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/392,312 filed Jun. 28, 2002, entitled “BondedMetal Components Having Uniform Thermal Conductivity Characteristics”,and which are incorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to composite bonded metalcookware, griddle plate or a sole plate for an iron and, moreparticularly, to bonded composite metal cookware, griddle plate or asole plate for an iron or the like, having at least one inner metallayer possessing a lower coefficient of thermal conductivity than theother metal layers of the composite so as to cause the heat to saturatein that layer prior to being transferred to the cook or ironing surface.In this manner, hot spots in the cook surface or iron are eliminated soas to improve the performance of the appliance and extend the life of anon-stick surface, if present.

2. Description of Related Art

It is well known in the art to manufacture multi-layered, compositebonded metal cookware of a variety of metals, most commonly aluminumand/or copper for good heat conductivity along with outer layers ofstainless steel for appearance, wear resistance and corrosionresistance. It is also known to employ a layer of a ferro-magneticmaterial, such as carbon steel or a 400 series ferritic stainless steelin place of the more common 300 series austenitic stainless steel if thecookware is to be used on an induction cooking device. All of thesecombinations are disclosed in the prior art as taught, for example, inUlam U.S. Pat. No. 4,646,935; in McCoy et al. U.S. Pat. No. 3,966,426;and Groll U.S. Pat. No. 6,267,830, all of which are incorporated byreference herein.

It is also well-known in the art to coat the cook surface or ironingsole plate surface with a non-stick material such as PTFE (“Teflon®”) orthe like. Hot spots may rapidly develop in cookware and sole plates forirons due to the use of conventional metals such as aluminum, copper andstainless steel alone or in composite form. These hot spots not only aretroublesome in cooking and ironing but they also cause an acceleratedthermal degradation of the non-stick surface. Even if no non-sticksurface is present, localized hot spots are undesirable in cookware,since it leads to unequalized cooking.

An attempt to minimize hot spots in cookware is disclosed in U.S. Pat.No. 4,541,411 to Woolf. A multi-ply cookware pan is disclosed by Woolfhaving inner and outer plies of aluminum or stainless steel enclosing anintermediate ply of a graphite material. The graphite material is notmetallurgically bonded to the adjacent metal layers but has thermallyanisotropic properties and is oriented so that its thermal conductivityis higher in a plane parallel to the cookware surface than it is in thedirection perpendicular to the surface to minimize hot spots on thecooking surface.

Since the graphite layer of Woolf is not metallurgically bonded to theadjacent aluminum and stainless steel layers, and because the aluminumand stainless steel layers are not themselves bonded along the cooksurface (due to the intermediate graphite ply), the resultant cookwareof Woolf would suffer certain shortcomings. First, due to the non-bondedgraphite ply, slight air gaps most probably would be present between thegraphite ply and the adjacent layers of aluminum and stainless steelwhich act as a thermal insulator by way of a barrier or film effect soas to lower the efficiency and uniformity of heat transfer across theinterface. In addition, due to the lack of metallurgical bonding betweenthe aluminum and stainless steel layers along the cooking surface of theWoolf cookware, one would expect some thermal warping to occur by virtueof the difference in thermal expansion coefficients of aluminum andstainless steel.

My invention overcomes the problems encountered in the prior art inattempting to eliminate hot spots and achieve more uniform heatingacross cook surfaces and other applications such as sole plates forironing (all of which are hereinafter collectively referred to merely as“cookware”). In addition, my invention increases the life of non-sticksurfaces by eliminating the hot spot problem of the prior art. Stillfurther, the heat-retardant layer of my invention contributes to theflatness of the cooking vessel during heat-up of multi-layer compositeswhich heretofore might be present due to differences in thermalexpansion coefficients of the several layers of different metals in thecomposite

SUMMARY OF THE INVENTION

Briefly stated, my invention provides composite, bonded metal cookwarehaving outer layers of aluminum and/or stainless steel with a coreconstruction comprising a layer of a heat-retardant metal such astitanium, titanium alloy, stainless steel or the like, bonded on bothsides to one of a layer of pure aluminum or a layer of Alclad aluminum.My invention also includes the method of making said bonded components.In one presently preferred construction, a titanium Ti 46 alloy striphaving a thickness of about 0.030-0.035 inch is bonded on both surfacesto strips of Alclad aluminum each having a thickness of about0.035-0.040 inch, with a layer of 304 stainless steel 0.015-0.017 inchbonded to each of the outer surfaces of the Alclad aluminum. Prior toroll bonding, the surfaces of the strips of the above materials aremechanically abraded by wire brush, wheel or the like to clean thesurfaces and expose bare, unoxidized metal. The sheets are stacked inthe following order: stainless steel layer—Alclad layer—titanium layer(or stainless steel layer)—Alclad layer—stainless steel layer; the stackmay then be heated or soaked to a temperature of 550° F.-600° F. in anoxygen-containing atmosphere (regular atmosphere), or at a highertemperature in an O₂-free atmosphere furnace. A temperature below about550° F. fails to provide a Ti bond during rolling, while a temperatureabove about 600° F. causes the formation of Ti oxides in anoxygen-containing furnace atmosphere. Such oxides prevent soundmetallurgical bonding.

The so-heated, ordered stack of sheets is then hot rolled in a firstpass in a rolling mill while at 550° F.-600° F. in the oxygen-containingatmosphere at a reduction of at least 5% up to 10% to achieve a bondbetween the Ti and Al, and between the Al and the outer stainless steellayers. The bonded pack can then be reheated if necessary and rolled ina second pass at a 10-20% reduction. The finished composite is thenpreferably heat treated at 650° F.-700° F. to improve the bondingstrength by way of diffusion bonding between adjacent layers.

The material so processed can then be blanked and formed by way ofdrawing in a conventional manner into the desired cookware shapes.

Comparative thermographic imaging of the composite material of thepresent invention indicates a uniformity of heating across the innercook surface which is free of hot spots.

If induction cookware is desired, a layer of ferro-magnetic material,such as carbon steel of a 400 series ferritic stainless steel, can beapplied to the outer (lower) surface of the cookware, i.e., that surfacewhich is closest to the inductor. In addition, a non-stick surface canbe applied to the cook surface or iron sole plate surface on thestainless steel layer or, alternatively, along an aluminum layer, ifdesired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of one presently preferred embodimentof a bonded metal composite according to the invention; and

FIGS. 2-3 are cross-sectional views of other presently preferredembodiments of the invention similar to FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

With reference now to the appended drawings, FIG. 1 schematicallydepicts, in cross section, one preferred embodiment of a bonded metalcomposite sheet 2 of the present invention. The composite sheet 2 is amulti-layered, rolled bonded construction comprising a layer 4 of amaterial having a lower coefficient of thermal conductivity than theother metal layers in the composite sheet 2. The presently preferredmaterial for the lower thermal conductivity layer 4 is titanium ortitanium alloy because of its relatively lower heat conductivitycoefficient compared with the aluminum coupled with its light weight,which is comparable to aluminum. Stainless steel has a coefficient ofheat conductivity similar to that of titanium and may be used for thelayer 4. Stainless steel is less expensive than titanium but is heavier,adding to the weight of the cooking vessel. In the composite sheet 2 ofFIG. 1, the titanium layer 4 is roll bonded on both sides betweenaluminum layers 6 and 6′. The aluminum layers 6 and 6′ comprise purealuminum of the 1100 series or the layers 6 and 6′ may be Alcladaluminum sheet. Alclad aluminum is made from an aluminum alloy core,such as 3003 alloy, roll bonded to outer layers of pure aluminum. The3003 aluminum alloy core provides improved strength while the outerlayers of pure aluminum provide good roll bonding properties. Forimproved roll bonding, the titanium layer (or stainless steel) 4 shouldbe bonded to a layer of pure aluminum. It will be understood that whenreference is made hereinafter to the titanium layer, that stainlesssteel could be substituted therefor, as well. The aluminum layers 6, 6′in the embodiment of FIG. 1 are, in turn, roll bonded to layers 7 and 8of stainless steel.

The stainless steel layer 8 defines the inner cook surface of thecomposite sheet 2 of FIG. 1 and is preferably made from an austeniticstainless steel such as type 304 stainless steel which offers goodcorrosion resistance and deep drawing properties. The outer stainlesssteel layer 7 is directly adjacent to the heat source and may also bemade from an austenitic grade of stainless steel such as 304 or fromaluminum (brushed, polished or anodized). If the cookware is to be usedfor induction type cooking, then the outer layer 7 is made from aferromagnetic material, such as carbon steel or from a ferriticstainless steel, or from a composite containing a ferritic stainlesssteel or carbon steel. A presently preferred ferritic stainless steelfor use in layer 8 is selected from the 400 series of stainless steelssuch as type 409 stainless steel. A roll bonded composite comprising alayer of a 400 series ferritic stainless steel sandwiched between layersof 300 series austenitic stainless steel may also be used for layer 8when the cooking vessel is intended for use with an induction cookingrange. Of course, such induction cookware can also be used withconventional gas or electric ranges.

Additional presently preferred embodiments of my invention are depictedin FIGS. 2 and 3 wherein the roll bonded composite sheets of myinvention are identified by reference numerals 20 and 200, respectively.In the one presently preferred embodiment of FIG. 2, the roll bondedcomposite sheet 20 comprises a layer of titanium or titanium alloy 40roll bonded to layers 60 and 60′ of aluminum or Alclad aluminum which,in turn, are roll bonded to layers of stainless steel 70 and 80. Thelower layer 70 may be one of austenitic or ferritic stainless steel or acomposite, i.e., a layer 70 of ferritic stainless steel with a furtherouter layer of austenitic stainless steel (not shown) roll bondedthereto, all as described above with reference to FIG. 1 and compositesheet 2.

The layer 90 of FIG. 2 defines the cook surface and is a non-sticksurface such as a Teflon®-brand PTFE or the like. Other surfaces such asTiN or ZrN ceramic nitride non-stick surfaces as disclosed in U.S. Pat.No. 6,360,423 to Groll, or a diamond surface applied by laser asdisclosed in U.S. Pat. No. 5,731,046 of Mistic et al. may also beemployed as surface 90. These patents are incorporated by referenceherein. The non-stick surface may be applied to the stainless steellayer 80 or, alternatively, directly to the aluminum layer 60′ in whichcase stainless layer 80 would be omitted as described below withreference to FIG. 3.

A further presently preferred embodiment of the invention is depicted asroll bonded composite metal sheet 200 in FIG. 3. In this embodiment alayer of titanium or titanium alloy 400 is roll bonded to layers 600 and600′ of pure aluminum or Alclad aluminum. The layer 600′ of aluminum orAlclad aluminum which is adjacent to the cook surface is coated with alayer 900 of non-stick material, as described above, applied thereto.The lower surface of the composite 200 closest to the heat source of thecooking range receives a layer 700 of austenitic or ferritic stainlesssteel. Alternatively, the outermost layer 700 may be a layer of rollbonded aluminum preferably having an anodized outer surface for scratchresistance and improved appearance.

In all of the embodiments of the roll bonded composites 2, 20 and 200discussed above, the respective titanium layers 4, 40 and 400 (orstainless steel layers) act to retard the heat transfer from the heatsource acting as surfaces 7, 70 and 700 to the cook surfaces 8, 90 and900. The titanium layers 4, 40 and 400 act as a “heat dam” or thermalbuffer and cause the temperature to conduct laterally or radially, asopposed to perpendicularly through the aluminum layers 6, 60 and 600.The heat is not conducted directly through the aluminum layers as inconventional composite cookware by virtue of the titanium layers. Inthis manner, the present invention prevents the occurrence of hot spotson the cooking surface. The titanium layer, by virtue of its lowercoefficient of thermal conductivity, acts as a thermal buffer and allowsthe heat to become more uniform along a radial direction in the aluminumlayers 6, 60 and 600 and then permits the heat to transfer by conductionuniformly through the aluminum layers 6′, 60′ and 600′ to the cookingsurface. In addition to avoiding hot spots along a horizontal cooksurface, the titanium layers 4, 40 and 400 provide for uniformtemperature distribution along the vertical sidewall of a deep drawn potor pan since the titanium layers form an integral part of the compositesheet 2, 20 and 200 which is used to draw the cookware.

Titanium is an ideal layer for the thermal buffer or heat dam layers 4,40 and 400 in cookware manufacture because titanium has a relatively lowdensity, comparable to aluminum, and, thus, does not add appreciableweight to the cookware. In addition, titanium roll bonds well to thealuminum and stainless steel layers and has a coefficient of thermalexpansion which is compatible with the other metals of the compositesheet 2, 20 and 200 so that thermally induced warping of the cookware isavoided.

Roll bonding the titanium layer 4, 40 and 400 to the adjacent aluminumlayers 6 and 6′, 60 and 60′, and 600 and 600′ assures a metallurgicallycontinuous bond across the entire composite (free of air gaps).Accordingly, the thermal transfer/conductivity across the bondedinterfaces between the aluminum layers 6, 60 and 600 and the titaniumlayers 4, 40 and 400 and, thence, between the titanium layers 4, 40 and400 and the respective aluminum layers 6′, 60′ and 600′ is uniform.

By way of example, and in no way limiting the scope of my invention, thecomposite sheet 2 of FIG. 1 may be made as follows. A pure titanium ortitanium alloy sheet such as a Ti—Al alloy, designated Ti-46 alloy, ofabout 0.035 inch thick is provided to form layer 4. Both surfaces of theTi sheet forming layer 4 are mechanically abraded using a wire brush orwheel or the like to expose the bare unoxidized metal. Two sheets ofAlclad aluminum forming layers 6 and 6′ having a thickness of between0.035-0.040 inch are also abrasively or wire wheel ground or abraded toexpose the underlying unoxidized pure aluminum and placed on both sidesof the titanium. The sheets 7 and 8 of type 304 stainless steel having athickness of about 0.015-0.017 inch are conditioned, cleaned and placedon either side of the Alclad aluminum layers 6 and 6′, respectively. Thestacked array of layers 7, 6, 4, 6′ and 8 is then heated in a regularatmosphere oven or furnace (containing atmospheric O₂) and heated to atemperature of between about 550° F. to 600° F. so that the stackedarray is uniformly soaked within that temperature range. A temperatureof below about 550° F. will provide an inadequate roll bond while atemperature in excess of about 600° F. causes the formation of Ti-oxide(in an O₂ furnace atmosphere) which also proves detrimental to rollbonding. Of course, if the heating and rolling is conducted in an O₂free atmosphere or environment, then preheating and rolling can beconducted at temperatures in excess of 600° F. Such O₂ free atmospheresor environments, however, add to the expense of the process, which maybe commercially unattractive.

The stacked array of sheets comprising layers 7, 6, 4, 6′ and 8 ispreheated within a temperature range of 550° F. to 600° F. and thenimmediately rolled (also in an O₂ atmosphere) in a rolling mill in afirst rolling pass of 5-10% reduction to effect a bond between thelayers of the array. The once hot rolled array is then subjected to asecond hot roll pass through the rolling mill at a further reduction ofbetween 10-20%. The thus rolled array is then at the desired finishthickness of about 0.100 inch, for example, and returned to a furnacefor a thermal treatment at 650-700° F. for about 4-8 minutes to improvethe diffusion bonding between the adjacent layers. This treatment causesa greater interatomic sharing of electrons between adjacent layers toprovide excellent bond integrity, uniformity and strength.

The roll bonded composite 2 is then processed in a conventional mannerto make cookware of desired configurations and sizes. The well-knownmanufacturing steps for making cookware involves the conventional stepsof blanking, drawing, buffing and the like and are in and of themselveswell-known in the metal working art.

The above-described layers 4, 40 and 400 forming the thermal buffer orheat dam, as mentioned, preferably consist of titanium, titanium alloyor stainless steel. The type of stainless steel is not particularlylimited and may be either a ferritic grade or an austenitic grade, forexample, of the 400 or 300 series, respectively, which offer goodformability. The ferritic grades, having ferromagnetic properties, arealso advantageous if the cooking utensil is to be used in connectionwith an induction heating range. Still other metals may also be used asthe head dam or thermal buffer layer 4, 40 or 400 as long as the othermetal has a coefficient of thermal conductivity lower than the otherhigh heat conductivity layer.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. The presentlypreferred embodiments described herein are meant to be illustrative onlyand not limiting as to the scope of the invention which is to be giventhe full breadth of the appended claims and any and all equivalentsthereof.

1. Cookware made from a bonded metal composite comprising: (a) a non-stick layer defining a cook surface of the cookware; (b) a layer of pure aluminum or Alclad aluminum bonded to layer (a); (c) a layer of stainless steel bonded to layer (b); (d) a layer of pure aluminum or Alclad aluminum bonded to layer (c); and (e) a layer of aluminum bonded to layer (d) defining an outer surface of the cookware, whereby layer (c) retards heat flow in a transverse direction to cause said layer (c) to distribute heat uniformly in a lateral direction to prevent hot spots from forming on the cook surface and thereby improve cooking performance and increase the life of the non-stick layer.
 2. The cookware of claim 1, wherein the aluminum layer (e) has an anodized surface.
 3. Drawn cookware made from a multi-layered composite having uniform thermal transfer properties, said multi-layered composite comprising a plurality of roll bonded metal layers including at least one inner layer of a metal having a coefficient of thermal conductivity lower than adjacent metal layers whereby said inner layer retards heat flow in a transverse direction to distribute heat in a lateral direction to thereby eliminate hot spots across a cook surface thereof and improve cooking performance.
 4. The cookware of claim 3 which includes a non-stick cook surface, wherein a life of said surface is extended due to the elimination of hot spots across the cook surface.
 5. Cookware formed from a multi-layered composite sheet having improved uniform thermal transfer properties, said composite sheet comprising a plurality of roll bonded metal layers including a core layer of stainless steel, roll bonded on both sides to immediately adjacent layers of pure aluminum or Alclad aluminum, inner and outer layers of stainless steel roll bonded to the layers of pure aluminum or Alclad aluminum, said core layer having a coefficient of thermal conductivity lower than said immediately adjacent aluminum layers whereby said core layer retards heat flow in a transverse direction to cause said core layer to distribute heat in a lateral direction to provide uniform heating across a cook surface of said cookware and wherein the outer layer of stainless steel is a ferromagnetic stainless steel whereby the cookware may be heated by induction.
 6. A method of making cookware made from a multi-layered composite metal sheet comprising the steps of: (a) providing a plurality of metal sheets including a first layer of stainless steel defining a cooking surface, a core layer comprising at least one sheet of stainless steel and first and second sheets selected from the group consisting of pure aluminum and Alclad aluminum facing said stainless steel core layer, a further sheet of a metal consisting of ferromagnetic stainless steel; (b) preparing said metal sheets by removing an oxide surface layer from surfaces thereof; (c) stacking said metal sheets to form an ordered array such that stainless steel core layer is sandwiched between the pure aluminum or Alclad aluminum sheets and the first sheet of stainless steel and the further sheet of ferromagnetic stainless steel faces one of the pure aluminum or Alclad aluminum sheets on opposite sides thereof; (d) heating said ordered array to a uniform rolling temperature; (e) rolling said ordered array to a desired thickness to form a roll bonded composite sheet; and (f) drawing said roll bonded composite to form cookware of a desired configuration comprising in an ordered array: a cooking surface formed by the layer of stainless steel, a layer of pure aluminum or Alclad aluminum, a thermal barrier layer of stainless steel, a layer of pure aluminum or Alclad aluminum and a layer of ferromagnetic stainless steel forming the outer surface of said cookware.
 7. The method of claim 6 wherein the metal sheets provided in step (a) include at least two sheets of Alclad aluminum which are stacked in step (c) on opposed sides of said core layer.
 8. A method of making cookware made from a multi-layered composite metal sheet comprising the steps of: (a) providing a plurality of metal sheets including a core layer comprising at least one sheet of stainless steel and at least two sheets selected from the group consisting of pure aluminum and Alclad aluminum; (b) preparing said metal sheets by removing an oxide surface layer from surfaces thereof; (c) stacking said metal sheets in an ordered array such that adjacent sheets having surfaces prepared from step (b) are facing each other and wherein the stainless steel sheet is sandwiched between the pure aluminum or Alclad aluminum sheets and forms a core layer of the ordered array; (d) heating said ordered array in a furnace or oven containing atmospheric oxygen to a rolling temperature of between 550° to 600° F.; (e) rolling said ordered array to a desired thickness to form a roll bonded composite sheet; and (f) forming said roll bonded composite sheet into cookware of a desired configuration.
 9. A method of making the cookware of claim 8 wherein the rolling step (e) comprises a first rolling reduction of at least 5% to about 10% followed by reheating to about 550°-600° F., rolling a second pass, and thereafter heat treating at about 650°-700° F. to improve bonding strength in the multi-layered composite metal sheet.
 10. The method of claim 8, including the step of applying a non-stick layer to a cook surface of the cookware.
 11. Cookware made from a bonded metal composite comprising: (a) at least one core layer of stainless steel; (b) upper and lower layers consisting of pure aluminum or Alclad aluminum, each layer roll bonded to upper and lower sides of said core layer; (c) a further layer of stainless steel roll bonded to the upper layer of the pure aluminum or Alclad aluminum to define a cook surface of said cookware; and (d) a further layer of stainless steel or aluminum roll bonded to the lower layer of pure aluminum, or Alclad aluminum to define an outer surface of said cookware, wherein the further layer of stainless steel is one of austenitic or ferritic stainless steel.
 12. The cookware of claim 11, wherein the further layer of stainless steel defining the outer surface of said cookware is ferritic stainless steel to provide for induction heating.
 13. The cookware of claim 11 wherein the further layer (c) of stainless steel cook surface has a non-stick layer applied thereto. 